you'll never be able to compare it to a prime 50, ain't gonna happen. No zoom is going to be able to outperform even the most humble of primes.

When using the cap, just make sure that you've pinched it as much as possible. I have problems with all of my lens caps - so this sounds normal to me. I like the center pinch caps more than the Canon stone-age edge pinch only ones. Do you have a filter? That could make matters worse.

If you're sure you're not front or back focusing, and you're just not impressed with the lens characteristics - send it back! Of course for your T1i the only other 2 good choices for sharpness would be the non VC tamron 17-50, or the EF-S 17-55. Of course it's debatable if these are any sharper, but at the edges wide open the Canon may have an advantage.